Disposable absorbent articles such as disposable diapers are well-known articles of manufacture which are worn by infants and incontinent persons. Disposable diapers are worn about the lower torso of the wearer and are intended to absorb and contain urine and other body exudates thereby preventing these exudates from soiling, wetting, or otherwise contaminating the articles (e.g. clothing, bedding, etc.) which come into contact with the diaper wearer.
When using a disposable diaper, the diaper user fits the diaper on the wearer and fastens it about the wearer's waist by a primary fastening system to thereby effect a side closure. Fitting the diaper about the wearer usually requires the front and back waist portions of the diaper to overlap each other. Since proper and sustained fit about the waist and legs of the wearer is vital for optimal performance in terms of minimizing leakage of body exudates out of the diaper, a diaper fastening system must be able to provide an effective side (or primary) closure in which the front and back waist portions are maintained in an overlapping configuration. As the diaper is worn, forces tend to cause the overlapping portions to shift position relative to each other. In other words, the overlapping portions are subjected to forces which tend to cause the portions to assume a position relative to each other which is different from the position they assume when the diaper is initially fitted to the wearer. Unless such shifting is limited, the fit and containment characteristics of the diaper are degraded as the diaper is worn. Thus, the fastening system must be designed to securely engage so it does not separate due to the peel forces and shear stress encountered by the fastening system during use.
As used herein, the "primary fastening system" refers to a system which provides the principal means for engaging and disengaging an article to be worn about a wearer. In the case of an absorbent article, such as a diaper, the primary fastening system operates as the side closure means which attaches the absorbent article about the wearer and provides a variable side closure to maintain the diaper on the wearer. This is to be distinguished from the waist closure fastening system described in U.S. Pat. No. 5,196,000 which is used in addition to a primary fastening system to provide lateral tension in order to prevent gapping, sagging, and rollover of the elasticized waistband. A primary fastening system encounters different amounts of shear and peel forces than such a waist closure system does and therefore must be designed differently to adequately resist those forces.
As used herein, the term "shear stress" refers to the distributed forces acting tangentially to the surface of contact of the members of the fastening system (or along the x/y plane). During the wearing of a diaper, shear stress tends to cause the members of the fastening system to shift with respect to each other. Shear stress is to be distinguished from "peel forces" which act on the members of the fastening system so as to separate and disengage from each other (in the z direction). A disposable diaper is typically subjected to peel forces in at least three ways. Peel forces are generated by the movements of the wearer during use as they tend to cause the first and second members of the fastening system to pull away from each other, by the wearer in trying to unfasten the fastening system during wear (this being a special problem for disposable diapers worn by infants because infants should not be able to unfasten and remove the diaper on their own), and by the user to check the diaper for soiling or to remove the diaper from the wearer. Because the fastening system should be able to be checked and removed by the user and because the user generated peel forces are much higher than the peel forces generated by the first two methods, the fastening system is preferably designed to have a resistance to peel forces (peel force resistance) with respect to only the movement and wearer generated methods. Therefore, the peel resistance should only be great enough to prevent failure of the fastening system during the first two methods but low enough to allow the user to check the diaper for soiling or to remove the diaper from the wearer without undue difficulty or tearing of other members of the diaper.
Therefore, it is desirable to design a primary fastening system capable of resisting shear stress and peel forces generated by the wearer but having a peel force resistance low enough to allow the user to easily remove the diaper or check the diaper for soiling.
Typically, fastening systems have been provided which have adequate shear force resistance to prevent the panels from shifting with respect to each other. However, because the shear forces are so high, the peel resistance of the fastening system is also very high. This is generally the situation because as the coat weight of the adhesive on the smooth surface of the backing web of the fastener is increased to improve the shear force resistance of the fastener, the peel force resistance also rapidly increases. The result is that typical fastening systems may rip the backsheet of the diaper during the process of unfastening it to check if the diaper has been soiled or to adjust its fit, thereby leaving a hole in the backsheet of the diaper and rendering the fastener unrefastenable and the diaper unusable, due to the high peel resistance of the fastener.
Other primary fastening systems have been provided but suffer from one or more problems. For example, hook and loop fastening systems such as those available commercially as Velcro.RTM. have been employed but are very expensive and require specific male and female engagement portions. Combination mechanical/adhesive primary fastener systems such as shown in U.S. Pat. Nos. 4,946,527 and 5,196,000 have also been employed. These systems suffer from being costly, from requiring specific matching male and female engagement portions and from the possibility of the adhesive being rendered unuseable by contamination from oils, ointments and powders used during the diapering procedure.
Thus, it would be advantageous to provide an inexpensive primary fastening system capable of using a wide variety of male and female engagement portions, resistant to contamination, having a high enough shear resistance to prevent the panels from shifting with respect to each other and sufficient peel resistance to prevent failure of the fastening system by the wearer's movements but a peel resistance low enough to allow the user to easily remove the diaper and check for soiling or fit without rendering the fastening system unrefastenable or the diaper unuseable.